Conventional concrete pavement installation involves preparing then positioning forms around an area intended for pavement. The forms have vertical inner surfaces to receive and contain poured concrete. The forms have horizontal top surfaces, which typically are level with the surface of the poured concrete, or, once cured, pavement surface. The forms have back surfaces that rest against appropriately-spaced stakes for holding the forms in place. To provide clearance for finish troweling, concrete workers often field cut chamfers between the top and back surfaces of the forms.
Very large pavements require substantial form preparation and positioning. This is especially true if stock materials for forms are short and/or flexible. Short and flexible forms require more staking than longer, more rigid forms to ensure true, unwavy pavement edges. Short forms also require more setup time for chamferring. Regardless of whether the forms are long or short, field chamferring requires considerable time for large pavement areas.
Ideally, the forms used for receiving poured concrete should have a true height for providing a true slab thickness. Unfortunately, forms in the field typically have a height that is less than a true height for an appropriate slab thickness. These forms of inadequate height typically may be positioned so that the top surfaces are at an appropriate height relative to the desired pavement surface height, but present bottom surfaces that do not contact, thus admit gaps through which poured concrete leaks. This wastes concrete and requires additional work to remove the excess portions.
Concrete leakage from the forms, especially at the butt joints, leaves depressions in a finished slab surface causing poor aesthetics. The depressions also impair surface coverings, such as tile, because the uneven surface promotes uneven or incomplete covering layout and adhesion. Cured leaked concrete also impinges on adjacent slabs causing voids and/or increasing the chances of obtaining a locked construction, which leads to cracks and joint failures. Finally, removing the cured excess typically damages the slab from which the excess is chiseled. Thus, avoiding form leaks is highly desirable.
Unfortunately, none of the foregoing provides a method of forming concrete and an apparatus for same that includes stiff, infinitely long, pre-chamferred forms with predetermined true height.
In construction of concrete pavements for highways, airport runways, large warehouse buildings and the like, preventing random cracking of the concrete necessitates dividing the pavement into convenient slab sections. To this end, concrete workers pour a monolithic concrete slab that is allowed to set for a short period. Then, the workers cut transverse grooves, having a depth on the order of one-fourth of the slab thickness, across the slab, with spacing between cuts selected in accordance with the application and design. Spacings from 12 to 40 feet are common for highway pavements.
As the concrete of the slab cures, forces derived from the exothermal curing reactions cause generally vertical cracks to develop through the slab thickness at the reduced cross-sections below each groove. This controlled cracking effectively divides the slab into predetermined separate slab sections.
The vertical cracks or joints define adjacent and interlocking faces formed by the cement and aggregates in the concrete. The interlocking faces transfer vertical shear stresses among adjacent slab sections, a phenomenon commonly referred to as “aggregate interlock,” as heavy objects, such as motor vehicles, pass over the joint.
Aggregate interlock causes wear among slab intersections with increasing use of the pavement. Additionally, cyclical and extreme temperature changes decrease slab volumes. Thus, over time, as traffic continuously passes over a joint, the intersections wear and become smooth, then fail altogether, resulting in relative vertical displacement of adjacent slab sections, hence a rough pavement surface. Joint failure also becomes increasingly susceptible to water intrusion, which may freeze and cause damage among adjacent slabs.
To discourage relative vertical displacement among adjacent slabs, prior art techniques provide for implanting dowels in concrete extending across the joint intersections. Some dowels are smooth steel rods with diameters on the order of one inch and lengths of two feet. Each rod is coated or otherwise treated so that it will not bond to concrete along its length or at least on one end thereof. Thus, as a slab expands and contracts during curing and subsequently with temperature changes, the dowel is free to move horizontally relative to, yet maintain vertical alignment of adjacent slabs, augmenting the aggregate interlock to transfer vertical shear stresses across the joints. See, for example, U.S. Pat. No. 3,397,626, issued Aug. 20, 1968, to J. B. Kornick et al. for Plastic Coated Dowel Bar for Concrete and U.S. Pat. No. 4,449,844, issued May 22, 1984, to T. J. Larsen for Dowel for Pavement Joints.
Among other problems, the foregoing techniques involve significant time and labor to produce and place the dowels.
Another technique to discourage relative vertical displacement among adjacent slabs involves embedding square-shaped load plates in adjacent slabs with opposed corners of the load plate aligned with the joint. To avoid shrink- or thermally-induced stress creation between the plate and a slab, concrete workers first embed a blockout sheath in one vertical joint face for receiving a load plate. To this end, the workers nail onto a form a mounting plate, from which a blockout sheath extends, then position the form to receive poured concrete. Once the concrete is cured and bonded to the blockout sheath, the workers remove the form board and leave the blockout sheath in place. Then the workers insert a load plate into the blockout sheath. Finally, the workers pour an adjacent slab, which bonds to the exposed portion of the load plate. See, for example, U.S. Pat. No. 6,354,760, issued Mar. 12, 2002, to Boxall et al., for System for Transferring Loads Between Cast-in-Place Slabs.
Drawbacks of the foregoing include the cost and labor associated with producing separate mounting and load plates, then assembling same following curing of a first concrete slab.
Referring to FIG. 13, a concrete floor 1100 typically is made up of a series of individual blocks or slabs 1102-1 through 1102-6 (collectively 1102). The same is true for sidewalks, driveways, roads and the like. Blocks 1102 provide several advantages, including relief of internal stress due to drying shrinkage and thermal movement. Adjacent blocks 1102 meet at joints 1104-1 through 1104-7 (collectively 1104). Joints 1104 typically are spaced so that each block 1102 has enough strength to overcome internal stresses that otherwise would cause random stress relief cracks. In practice, blocks 1102 should be allowed to move individually, but also should be able to transfer loads from one block to another block.
Transferring loads between blocks 1102 usually is accomplished with smooth steel rods, also referred to as dowels, embedded in two blocks 1102 defining joint 1104. For instance, FIG. 14 shows a side view of dowel 1200 between slabs 1102-4 and 1102-5. FIG. 15 shows a cross-sectional view along line XV-XV in FIG. 14 of several dowels 1200 spanning joints 1104 between slabs 1102. Typically, a dowel or bar 1200 is approximately 14 to 24 inches long, has either a circular or square cross-sectional shape, and a thickness of approximately 0.5-2 inches. Such circular or square dowels are capable of transferring loads between adjacent slabs 1102, but have several shortcomings.
U.S. Pat. Nos. 5,005,331, 5,216,862 and 5,487,249, issued to Shaw et al., which are incorporated herein by reference, disclose tubular dowels receiving sheaths for use with dowel bars having circular cross-sections.
Referring to FIG. 16, a shortcoming of circular or square dowels is that if dowels 1200 are misaligned, or not perpendicular to joint 1104, they can undesirably lock the joint together causing unwanted stresses that could lead to slab failure in the form of cracking. Such misaligned dowels can restrict movement in the directions 1400-1 and 1400-2.
Another shortcoming of square and round dowels is that they typically allow slabs to move only along the longitudinal axis of the dowel. As shown in FIG. 17, movement is allowed in direction 1500, parallel to dowels 1200, while movement in other directions 1502-1 and 1502-2, and directions into and out from the page is restrained. Such restraint of movement in directions other than parallel to the longitudinal axes of dowels 1200 could result in slab failure in the form of cracking.
U.S. Pat. No. 4,733,513 ('513 patent) issued to Shrader et al., which is incorporated herein by reference, discloses a dowel bar having a rectangular cross-section and resilient facings attached to the sides of the bar. As disclosed in column 5, at lines 47-49 of the '513 patent, such bars, when used for typical concrete paving slabs, would have a cross-section on the order of ½ to 2-inch square and a length on the order of 2 to 4 feet.
Referring to FIGS. 18 and 19, yet another shortcoming of prior art dowel bars is that, under a load, only the first 3-4 inches of each dowel bar transfers the load. This creates very high loadings per square inch at the edge of slab 1102-2, which can result in failure 1600 of the concrete below dowel 1200, as shown in FIGS. 18 and 19. Such a failure also could occur above dowel 1200.
Unfortunately, none of the foregoing provide a method of forming concrete and an apparatus for same that includes partially coated load plates carried in slotted forms.
What are needed, and not taught or suggested in the art, are a method of forming concrete and an apparatus for same that provide partially coated load plates carried in pre-slotted, stiff, infinitely long, pre-chamferred forms with predetermined true height that: (1) increase relative movement between slabs in a true direction parallel to the longitudinal axis of the joint; (2) reduce loadings per square inch close to the joint; (3) maximize material at the joint for transferring loads between adjacent cast-in-place slabs efficiently; (4) minimize raw materials needed in a load plate; and (5) promote exact load plate positioning to foster better perpendicular and parallel alignment with the joint and upper concrete surface.